Lost here. Can someone explain why this works

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pucho812

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I am running the following circuit in the link. I have the 24v version and have a wall wart  for the power. If I take 24v from a different source  and apply it to the control pins, it works. The relays will switch and switch back when the control source power  is moved to a different control connection.  But trying to grasp how it is working. While it does the job, why does it do the job?

https://images-na.ssl-images-amazon.com/images/I/A1edk4gzRTL.pdf
 
I don't understand. What exactly are you confused about? You apply a few volts to an input, it turns on the transistor and grounds the relay coil engaging it.
 
Think of the base of the transistor as a dc input rather then an audio one. As long as you apply dc (signal) between the base and ground it will operate the transistor and hence the relay.

Cheers

Ian
 
magic?

It is a pretty basic common emitter switch, using a darlington (two transistors in series) switch so very high current gain...  A tiny base current results in a large collector current that saturates the output hard on...

The input has resistor divider network so input voltage (like 5V logic) provides a safe current to the darlington base to turn on the switch..

JR
 
> What lost me is I am running vcc from a wall wart for this  but getting the 24v for control signal from a different source then the wall wart.

All electrons look alike. How can your transistor know where they came from?

My cooling uses a 6V battery to switch a 24VAC relay which switches another 24V relay to clank the contactor and apply 240VAC to the freon squeezer.
 
pucho812 said:
What lost me is I am running vcc from a wall wart for this  but getting the 24v for control signal from a different source then the wall wart.
That should be ok but note that it would be very easy to feed a switch 24V through a resistor put a switch on the end and not connect the grounds of two things together unnecessarily. Otherwise, if you're not using a simple switch, just make sure you connect grounds together at the relay module. Presumably the wall-wart ground is floating so it will adjust to match the CV ground just fine. You might want to measure for DC between the grounds just as a sanity check in case neutral is connected to ground and the grounds are off a bit. If they are off even a little you could get some significant current on the ground line which would be bad. Also, if the CV wire is really long (like a few meters or so), you might consider using a shielded cable, add a 100R at the source and a small cap on the relay module input to minimize the possibility of triggering the relay from some kind of EMI like when you flip on a nearby florescent light (although this is almost certainly unnecessary with a bipolar switch). Otherwise, you just need to apply enough voltage to the input to overcome the two diode drops of the darlington albeit through the divider (datasheet says 2.5V).
 
pucho812 said:
What lost me is I am running vcc from a wall wart for this  but getting the 24v for control signal from a different source then the wall wart.
That is one of the basic underlying characteristics of digital logic. Your transistor only cares about getting a digital 1, voltage positive wrt ground.  Digital logic is not very picky about how positive or where it comes from (within reason).

JR
 
JohnRoberts said:
It is a pretty basic common emitter switch, using a darlington (two transistors in series) switch so very high current gain...  A tiny base current results in a large collector current that saturates the output hard on...
I would just add here, if you don't mind, that sometimes using darlington transistor as switching element isn't good idea because the saturation  voltage Vcesat is relatively high (Vcesat(one transistor) + Vbe(second) ) so the disipation on the switching darlington can be substantial.
The voltage loss on darlington transistor can be critical also. For example, it can't be used for switching 3V relays supplied from 3V power supply.  A  singe transistor with Vcesat=0.1V can be used.
 
moamps said:
I would just add here, if you don't mind, that sometimes using darlington transistor as switching element isn't good idea because the saturation  voltage Vcesat is relatively high (Vcesat(one transistor) + Vbe(second) ) so the disipation on the switching darlington can be substantial.
perhaps TMI but they make and routinely use darlington transitor arrays for GP switching. These are normally expecting 5V logic but the first array I checked will switch 300mA from 3V input and 200mA with 2.4V input.
The voltage loss on darlington transistor can be critical also. For example, it can't be used for switching 3V relays supplied from 3V power supply.  A  singe transistor with Vcesat=0.1V can be used.
indeed but will require a lower Z input resistor to get enough base current.

JR
 
Operating voltage depends of relay's rating and vice versa.
Control input voltage is in ranges
0 to 0,5V for off,
0,5 to 2,5 not defined and
2,5 to 24V for on state.
So you can use any control voltage from 2.5 to 24V for switching relays to on state.
 

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pucho812 said:
this may be elementary but I'll ask....

Why do they require the input to be up to 24V?
They don't , they accept up to 24v

Some times these auxiliary relays get driven by open collector tallies... a pull up R to 24V supply will work and not exceed input spec.. but that seems arbitrary to not confuse digital engineers...  ::)

JR
 
well wired in and it is working.

nothing gets warm to touch or anything. Everything is grounded to chassis including the the wall wart, PCB grounds are tied together. The input trigger voltage comes from the desk.
The desk had a module in it which was a speaker select module but when they moved the desk from the old location to the new location, the original install techs could not  figure out how to wire it and may have hacked off some critical things.  I have no schematics of that section so I don't know for sure. It has  relays in a card cage that is seperate and the relays are supposed to switch on and off when you hit the buttons on the speaker select module.  for a few years off and on several other techs could't figure it out. At one point the owner was asked to hit up the designer who did all the desk mods and that never happened.    The owner has no idea and would go, it just worked. Famous last words I say.  Through testing I was able to get 24V from the desk down to 1 of 4 eco pins depending on which speaker was selected(mains, mids, small,  smallest), so used that as my input trigger. So far it is working and nothing has blown up or gets hot.  the selects  with new card works. Ha za Ha za. thanks for all the insight and help guys. You are a true delight...
 
Phrazemaster said:
I remember you showing me that desk Pucho, so kudos for getting this sorted!

Mike
the first design was on vero board and worked ok, but the jbox and switches looked too low fi as you May remember. So a new pcb  that is made elsewhere and wiring to the buttons got it working as intended.
 
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